Multilayer systems should be understood to include systems with special cover layer(s) (cap layer(s)).
Electric field intensity is defined here as the time-averaged square of the electric field strength, photocurrent as a photo-induced electric current occurring due to radiation, photoemission as the photo-induced release of electrons into the vacuum occurring due to radiation, photoelectrons as the photo-induced electrons released into the vacuum due to radiation, and secondary electrons as electrons retarded via secondary processes. Optical elements, e.g., photomasks or multilayer mirrors, for the extreme ultraviolet wavelength range (EUV), particularly wavelengths of between 11 nm and 14 nm, are used in EUV lithography of semiconductor components. Typical EUV lithography devices have eight or more mirrors. To achieve, nevertheless, a sufficient total intensity of the working radiation, the mirror reflectivities must be as high as possible since the total intensity is proportional to the product of the reflectivities of the individual mirrors. To the extent possible, the mirrors should retain this high reflectivity over their entire life. Furthermore, the homogeneity across the mirror surfaces must be retained over the entire life of the mirrors.
The reflectivity and the life of EUV mirrors and EUV photomasks are affected particularly by the contamination of the surface under EUV radiation in the form of carbon deposits and by oxidation of the surface. Deposits are due, for example, to carbon-containing substances released in the form of gases from some of the device components or from the photoresist with which the wafers to be radiated are coated. Oxidation is caused by oxygen-containing molecules present in the residual gas atmosphere, which are split into radicals by the EUV radiation by means of photoemission. Photoelectrons emitted from the surface of the optical element lead to an accumulation of contamination because they split molecules of the residual gas adsorbed to the surface in a dynamic equilibrium into fragments, which are then permanently deposited on the surface or react therewith.
In the residual gas atmosphere, hydrogen, oxygen, nitrogen, water vapor, carbon monoxide and carbon dioxide typically dominate. As a rule, the partial pressures in mbar fall within the following ranges:
Typical range for ultra-Typical bakedTypical non-bakedhigh vacuum systemsvacuumvacuum[mbar][mbar][mbar]CxHy10−12-10−810−1210−8CO10−12-10−810−1210−9CO210−12-10−810−1210−9H210−10-10−710−1110−8H2O10−10-10−610−1110−7N210−10-10−710−1110−7O210−10-10−710−1110−8
Depending on the partial pressure conditions and the incident EUV intensity, either the process of surface oxidation or the process of the deposition of carbon on the surface predominates. Thus, one speaks either of an oxidizing or a carbonizing environment.
German Laid Open Publication DE 41 06 841 A1 discloses a method for determining contamination by means of photoemission, in which photoelectrons are released by sequentially illuminating a surface and are detected with an electron collection device. The photoelectric current can be used to deduce the thickness of the contaminant layer. To compensate the effects of capacitive current, a second electron collection device is provided, which is adjacent to the first electron collection device and spaced at the same distance from the surface. The currents detected by the electron collection devices are subtracted from one another. Furthermore, means are provided to compensate the effects of photovoltaic current.
U.S. Pat. No. 6,004,180 is concerned with the cleaning of electron-emitting components forming the cathode of the cathode ray tube. Introducing a gas, e.g. oxygen, causes the deposits on the cathode to be converted into gaseous products. The document does not describe a specific control of the gas intake.
From the Japanese publication JP 62051224 A it is known to track the cleaning with UV-light-supported oxidation by measuring photoelectrons. Surfaces contaminated with carbon-containing substances are exposed to UV light in an oxygen-containing atmosphere. Under the UV influence, ozone forms, which in turn is split into oxygen radicals, which etch the carbon contaminants off the surface to be cleaned. The UV light furthermore excites the emission of secondary electrons. Their number increases with decreasing thickness of the contaminant layer. The intensity of the photoemission is signaled. This signal, however, is not used to control a cleaning process.
European Publication EP 0 987 601 A2 discloses a method and a device for measuring the degree of contamination of optical elements within a soft X-ray lithographic system. For this purpose, the photoelectrons emitted from the potentially contaminated surface are measured. This signal is evaluated online. It is translated into a contamination level by comparing it with empirical data. If a specific contamination level is reached, the corresponding optical element must be replaced.